I think I'm qualified to answer this question as I was conferenced out. That's probably what you meant by “failure”. However, the amount of philosophy you learn in 5 days of stay in your SSB centre will most definitely not let you go with a feeling of “failure”. Anyway coming to your question.
Firstly, people must have told you bullshit like try to “lead” the group in all the tasks, speak more in GD, show them that you are a leader in everything you do. This is a very wrong idea and this is exactly what I did. I had good speaking ability and I tried to speak wherever I could, mistake number one. When it came to group discussions, I spoke a lot in both the rounds, and I spoke well. However, the person who got selected finally was the one who didn't interrupt ANYONE while they were speaking and gave JUST 2 valid points in each GD without getting excited about it. This in no way means that you have to give exactly 2 points. It implies that if you have something to say, say it. Otherwise don't. DO NOT REPEAT YOUR ARGUMENTS EVEN IF OTHER CANDIDATES SAY THE SAME THING OVER AND OVER AGAIN. The quality of officers is to LISTEN. Most of the candidates realize it when it's too late (including me), or they don't at all. So If someone says something, listen.
Coming to the regular stuff, you should know currencies and capitals of a major countries and also some global issues like terrorism and stuff like that. Also know political map of India throughly. Your writing skills must be good and make sure that you know yourself completely before you reach the SSB centre. Know what you like and dislike and why. Never be unsure about yourself, this is a major drawback.
One more thing, interview will be very very friendly and interviewer will try to know the most out of you in a sweet way. Make sure you don't lie, as sooner or later you will be caught. Also, don't project anything that happened to your life as a trauma, let everything be a good learning experience. Make them understand that you have grown from every mistake you have committed and every situation you've been in, this was my mistake number 2. I gave them a feeling that I regretted some of my actions, which isn't right. Thirdly, do not try to act witty by answering everything right as the interviewer stops. You can have a few seconds thought to organize your answer, that's how a mature person speaks.
Do make sure that you make friends with everyone in your batch, it will boost your confidence in group discussions and help deliver your talk efficiently in the later round.
Lastly, please don't ignore the questions and other psychological tests that will be carried out during the SSB (including the screening which is the most important part to get to the next round). Everything has its own weightage. These were some of the things I felt are necessary qualities and some of which I lacked back then. But as I told you, if, and I hope not, you get rejected, you'll ponder upon it and soon realize your mistakes and correct them. It will be a great experience. All the best for it.